1. Field of the Invention
The present invention relates to a nonaqueous electrolytic solution and a nonaqueous electrolyte secondary battery using the solution.
2. Description of the Related Art
In a lithium-ion secondary battery using a nonaqueous electrolytic solution (that is, nonaqueous electrolyte secondary battery: or referred to “battery”, hereinafter), if the battery is overcharged using a voltage larger than a normal operating voltage (normally, 4.2V at full charge if LiCoO2 is used as a cathode), excess lithium-ions may be released from a cathode, and simultaneously an excess amount of a lithium metal is generated in an anode to produce a dendrite thereof. Accordingly, the cathode and the anode become chemically unstable, whereby carbonates generally included in the nonaqueous electrolytic solution may react with the lithium metal to undergo the decomposition, resulting in causing a rapid exothermic reaction. The exothermic reaction may excessively heat the whole battery, thereby to spoil the safety of the battery.
Generally, a battery comprises a protection circuit or the like so that such a protection circuit prevents the battery from being overcharged. As a result, the above mentioned accident may not be happened. However, assuming that a charger or a protection circuit goes out of order, it is demanded to secure the high safety of the battery, even if the battery is overcharged. In particular, such a demand is growing more and more as long as an energy density or a capacity of the battery is increasing.
For responding to the demand as mentioned above, a method for securing the safety against the overcharge is disclosed, in which a small amount of an aromatic compound is added to an electrolytic solution as an additive agent. See the Japanese Patent Publication No. 3275998, the Japanese Laid-Open Patent Publication Nos. H09-171840, H10-321258, and H07-302614, and Electrochemical and Solid-State Letters, 9(1), A24-A26 (2006). More specifically, for example, the Japanese Patent Publication No. 3275998, the Japanese Laid-Open Patent Publication Nos. H09-171840 and H10-321258 disclose that the overcharge of the battery is suppressed by operating an internal electricity shutdown device through generating a gas, or generating a conductive polymer, inside the battery when overcharged. Herein, the method is conducted by using cyclohexylbenzene, biphenyl, 3-R-thiophene, 3-chrolothiophen, and furan or the like which are dissolved in the electrolytic solution. Further, the Japanese Laid-Open Patent Publication No. H07-302614 discloses that an anisole derivative with MW of 500 or less is used as an additive agent for the electrolytic solution. Further, the overcharge may be suppressed, if an electroactive thiophene based polymer with several thousands MW such as poly(3-butylthiophene) and poly(3-phenylthiophene) are added to the electrolytic solution, as disclosed in Electrochemical and Solid-State Letters, 9(1), A24-A26 (2006).
Herein, each compound such as cyclohexylbenzene disclosed in the Japanese Patent Publication No. 3275998, the Japanese Laid-Open Patent Publication Nos. H09-171840, H10-321258, and H07-302614, and Electrochemical and Solid-State Letters, 9(1), A24-A26 (2006), has the effect of suppressing the overcharge of the battery through the electrolytic polymerization thereof in the nonaqueous electrolytic solution. However, after the compound is completely consumed through the electrolytic polymerization in the nonaqueous electrolytic solution, the overcharge of the battery may be caused again. In such a case, if the product formed through the electrolytic polymerization of the above mentioned compound has the effect of increasing the internal resistance of the battery, the effect of suppressing the overcharge may be enhanced. However, the product formed through the electrolytic polymerization of each compound described in the Japanese Patent Publication No. 3275998, the Japanese Laid-Open Patent Publication Nos. H09-171840, H10-321258, and H07-302614, and Electrochemical and Solid-State Letters, 9(1), A24-A26 (2006) respectively, has little effect of increasing the internal resistance of the battery.
Further, thiophene derivatives such as 3-R-thiophene and 3-chlorothiophene are electrochemically unstable, thereby to be easily decomposed in the battery, resulting in the decrease of the battery performance. In particular, thiophene undergoes the electrolytic polymerization at 4.0V or less at the lithium metal standard voltage. Accordingly, it is very difficult to use thiophene for the battery of which operating voltage is generally 4.0V or more.